The quality and performance of a digital fiber optic transmitter is determined by the distance over which its signal can propagate without severe distortions. The Bit Error Rate (BER) of the signal is measured at a receiver after propagation through dispersive fiber, and then the optical power required to obtain a certain BER (typically 10−12), sometimes called the sensitivity, is determined.
The difference in sensitivity between (i) the output of the transmitter and (ii) the sensitivity after propagation, is sometimes called the dispersion penalty. This sensitivity difference is typically characterized in connection with the distance over which a dispersion penalty reaches ˜1 dB. A standard 10 Gb/s optical digital transmitter, such as an externally modulated source, can typically transmit up to a distance of ˜50 km in standard single mode fiber at 1550 nm before it reaches a dispersion penalty of ˜1 dB; this distance is sometimes called the dispersion limit. This limit is determined by the fundamental assumption that the signal is transform limited, i.e., the signal has no time varying phase across its bits and has a bit period of 100 ps, or 1/bit rate.
Another measure of the quality of a transmitter is the absolute sensitivity after fiber propagation.
A system for long-reach lightwave data transmission through optical fibers is disclosed in (i) U.S. patent application Ser. No. 10/289,944, filed Nov. 6, 2002 by Daniel Mahgerefteh et al. for POWER SOURCE FOR A DISPERSION COMPENSATION FIBER OPTIC SYSTEM; (ii) U.S. patent application Ser. No. 10/680,607, filed Nov. 6, 2003 by Daniel Mahgerefteh et al. for FLAT DISPERSION FREQUENCY DISCRIMINATOR (FDFD); and (iii) U.S. patent application Ser. No. 10/308,522, filed Dec. 3, 2002 by Daniel Mahgerefteh et al. for HIGH-SPEED TRANSMISSION SYSTEM COMPRISING A COUPLED MULTI-CAVITY OPTICAL DISCRIMINATOR; which patent applications are hereby incorporated herein by reference. The transmitter associated with this novel system is sometimes referred to as a Chirp Managed Laser (CML™) by Azna LLC of Wilmington, Mass. In this new system, a frequency modulated source is followed by an Optical Spectrum Reshaper (OSR) which converts frequency modulation to a substantially amplitude modulated signal and partially compensates for dispersion in the transmission fiber. The Optical Spectrum Reshaper (OSR), sometimes referred to as a frequency discriminator, can be formed by any appropriate optical element that has a satisfactory wavelength dependent transmission function. The OSR can be adapted to convert frequency modulation to amplitude modulation.